This invention relates to a protective apparatus for a power circuit and, more particularly, to a solid state overload relay apparatus that will generate a trip signal in response to a selected set of predetermined conditions, such as an overload, underload or a loss of a phase, in the power circuit.
Typically, AC magnetic contactors and starters are electric controllers used for accelerating a motor from its rest mode to a normal operating speed. During this start-up period and during normal operating conditions, electric motors may be subject to harmful overloads, loss of a phase or excessive underload conditions. Any one of these conditions, if it persists for a predetermined time, may be harmful to the electric motor or its operation and, therefore, a solid state overload relay apparatus, which incorporates circuitry to handle one or all of the three above-mentioned conditions, would be desirous.
One such apparatus having circuitry for detecting overload currents in a motor power circuit is disclosed in U.S. Pat. No. 3,851,216, assigned to the assignee of this invention, which employs one type of discrete component circuitry for providing overload protection in power circuits of electric motors. However, the overload relay apparatus of that invention included only discrete elements to provide temperature compensation for the circuitry. Next, the overload and reset features functioned separately, rather than as an integral circuit.
Other prior art includes U.S. Pat. No. 4,246,622 Hosoda, et al and U.S. Pat. No. 4,041,540 to Kampf, et. al.
Some of the foregoing limitations found in prior art protective devices for motor power circuits pertaining to temperature compensation, overload/reset functions, and a single protective apparatus module covering the full range of motor currents for a single NEMA size starter are substantially solved by the circuitry of the present invention. In the present invention, the trip output signals from the overload detection circuit, the underload detection circuit, the single phase detection circuit and the attendant reset circuit for the protective apparatus are all summed at a common point and fed to a negative input of a comparator which acts as a Schmidt trigger to operate a pair of electromechanical relays having overload and alarm contacts, respectively.
An object of the present invention is to provide a solid state overload relay apparatus for electric motors in which the entire circuitry closely approximates a zero temperature coefficient, rather than having discrete components for the temperature compensation function.
Another object is to provide a solid state overload protective apparatus for electric motors which has several functions, such as overload, underload and single phase detection circuits and a reset circuit, all having their trip signal outputs and reset signal outputs, respectively, summed at a common point to provide an input to a two level comparator for trip and rest of the overload and alarm relays.
Still another object is to provide a solid state overload relay protective apparatus which automatically opens the overload contacts and momentarily closes the alarm contacts when a power failure in the voltage supply occurs for greater liability of the protective apparatus.
A further object is to provide a solid state overload relay protective apparatus in which a control therein includes a variable adjustment so that the entire current ranges of a single NEMA size starter can simply be dialed to achieve the motor full load current setting.
In accordance with the present invention, a solid state overload relay protective apparatus for electric motors which includes not only the overload feature but the additional features of underload and single phase protection comprises small current transformers for sensing the current flow in each phase of the motor, like those found in ground fault sensing art. The current transformers generating an output signal proportional in magnitude to the sensed current are connected to a current-to-voltage converter circuit. The output signal of the converter is then fed to a scaling and summing amplifier. The output signal of the scaling and summing amplifier is fed through an ideal diode peak detector which, in turn, provides an input signal to a time integrator amplifier. The output signal from the time integrator amplifier is fed through a diode to one input of a comparator acting as a Schmidt trigger. When the output signal of the time integrator amplifier exceeds a predetermined trip reference voltage signal at the other input of the comparator, the output state of the comparator changes to a trip level, removing the base drive from a driver transistor de-energizing the coil of the overload relay and opening its contacts. Moreover, this circuit construction of the present invention permits an underload detection circuit and a single phase detection circuit also to be connected to the negative input of the comparator so that a single phase, or underload, condition exceeding predetermined limits also will change the output state of the comparator to the trip level and, thus, remove energy from the relay coil operating the overload contacts.